You say that they nested. Did they ever have eggs? If not, you may have two males with the second male now becoming more dominant/aggressive. If eggs were laid and never hatched, they may have been infertile because you have 2 females.

Yes, it is possible that if the male is a male, he became somewhat subdued, but not likely especially around predators. He could become skittish and seek to go into deep water than fight if he is scared or confronted by a predator, but usually, the males become even more aggressive. It is for this reason that we think you may have a different gendered swan than you originally thought. The Regal Swan